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SIIT LARGE CAP INDEX FUND  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading SIIT LARGE CAP INDEX FUND
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Investment results that correspond to the aggregate price and dividend performance of the securities in the Russell 1000 Index.
Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy and hold Fund shares.
Operating Expenses Caption [Text] rr_OperatingExpensesCaption ANNUAL FUND OPERATING EXPENSES (expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading PORTFOLIO TURNOVER
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual Fund operating expenses or in the Example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 16% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 16.00%
Expense Example [Heading] rr_ExpenseExampleHeading EXAMPLE
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund's operating expenses remain the same.
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs and returns may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

The Large Cap Index Fund invests substantially all of its assets (at least 80%) in securities listed in the Russell 1000 Index. The Russell 1000 Index (the Index) measures the performance of the large-cap segment of the U.S. equity universe and includes approximately 1000 of the largest securities based on their market capitalization. The Fund's investment performance will depend on the Fund's tracking of the Index and the performance of the Index. The Fund's ability to replicate the performance of the Index will depend to some extent on the size and timing of cash flows into and out of the Fund, as well as on the level of the Fund's expenses.


The Fund's sub-adviser (the Sub-Adviser) selects the Fund's securities under the general supervision of SEI Investments Management Corporation (SIMC or the Adviser), but the Sub-Adviser makes no attempt to "manage" the Fund in the traditional sense (i.e., by using economic, market or financial analyses). Instead, the Sub-Adviser generally will attempt to invest in securities composing the Index in approximately the same proportions as they are represented in the Index. It may not be possible or practicable to purchase all of the securities composing the Index or to hold them in the same weightings as they are represented in the Index. In those cases, the Sub-Adviser may employ a sampling or optimization technique to replicate the Index. In seeking to replicate the performance of the Index, the Fund may invest, to a lesser extent, in American Depositary Receipts (ADRs). The Fund may also invest in securities of companies located in developed foreign countries and securities of small capitalization companies. The Sub-Adviser may, but is not required to, sell an investment if the merit of the investment has been substantially impaired by extraordinary events, such as fraud or a material adverse change in an issuer, or adverse financial conditions. The Fund's return may not match the return of the Index. The Index's market capitalization range and the composition of the Index are subject to change.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

Market Risk — The risk that the market value of a security may move up and down, sometimes rapidly and unpredictably. Market risk may affect a single issuer, an industry, a sector or the equity market as a whole.


Large Capitalization Risk — The risk that larger, more established companies may be unable to respond quickly to new competitive challenges such as changes in technology and consumer tastes. Larger companies also may not be able to attain the high growth rates of successful smaller companies.


Investment Style Risk — The risk that its investment approach, which attempts to replicate the performance of the Russell 1000 Index, may underperform other segments of the equity markets or the equity markets as a whole. The Fund is also subject to the risk that large capitalization securities may underperform other segments of the equity markets or the equity markets as a whole.


Tracking Error Risk — The risk that the Fund's performance may vary substantially from the performance of the benchmark index it tracks as a result of cash flows, Fund expenses, imperfect correlation between the Fund's investments and the index's components and other factors.


Sampling Risk — The Fund may not fully replicate the benchmark index and may hold securities not included in the index. As a result, the Fund may not track the return of its benchmark index as well as it would have if the Fund purchased all of the securities in its benchmark index.


Depositary Receipts Risk — Depositary receipts, such as ADRs, are certificates evidencing ownership of shares of a foreign issuer that are issued by depositary banks and generally trade on an established market. Depositary receipts are subject to many of the risks associated with investing directly in foreign securities, including, among other things, political, social and economic developments abroad, currency movements and different legal, regulatory and tax environments.


Foreign Investment Risk — The risk that non-U.S. securities may be subject to additional risks due to, among other things, political, social and economic developments abroad, currency movements, and different legal, regulatory and tax environments.


Small Capitalization Risk — Smaller capitalization companies in which the Fund may invest may be more vulnerable to adverse business or economic events than larger, more established companies. In particular, small capitalization companies may have limited product lines, markets and financial resources and may depend upon a relatively small management group. Therefore, small capitalization stocks may be more volatile than those of larger companies. Small capitalization stocks may be traded over-the-counter or listed on an exchange.


Liquidity Risk — The risk that certain securities may be difficult or impossible to sell at the time and the price that the Fund would like. The Fund may have to lower the price of the security, sell other securities instead or forego an investment opportunity, any of which could have a negative effect on Fund management or performance.


Investing in the Fund involves risk, and there is no guarantee that the Fund will achieve its investment goal. You could lose money on your investment in the Fund, just as you could with other investments. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money [Text] rr_RiskLoseMoney You could lose money on your investment in the Fund, just as you could with other investments.
RIsk Not Insured [Text] rr_RiskNotInsured An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years, and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception compare with those of a broad measure of market performance. The performance information shown is based on full calendar years. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. For current performance information, please call 1-800-DIAL-SEI.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years, and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception compare with those of a broad measure of market performance.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-DIAL-SEI
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

Best Quarter: 16.46% (6/30/09)
Worst Quarter: -14.64% (09/30/11)
The Fund's total return from January 1, 2019 to June 30, 2019 was 18.80%.
Year to Date Return, Label rr_YearToDateReturnLabel total return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2019
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 18.80%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 16.46%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (14.64%)
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2018)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deduction for fees, expenses or taxes)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Your actual after-tax returns will depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher In the event of negative performance, the Fund's returns after taxes on distributions and sale of Fund shares are calculated assuming that an investor has sufficient capital gains of the same character from other investments to offset any capital losses from the sale of Fund shares. As a result, the Fund's returns after taxes on distributions and sale of Fund shares may exceed the Fund's returns before taxes and/or returns after taxes on distributions.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. In the event of negative performance, the Fund's returns after taxes on distributions and sale of Fund shares are calculated assuming that an investor has sufficient capital gains of the same character from other investments to offset any capital losses from the sale of Fund shares. As a result, the Fund's returns after taxes on distributions and sale of Fund shares may exceed the Fund's returns before taxes and/or returns after taxes on distributions.
SIIT LARGE CAP INDEX FUND | Russell 1000 Index Return (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Russell 1000 Index Return (reflects no deduction for fees, expenses or taxes)
1 Year rr_AverageAnnualReturnYear01 (4.78%)
5 Years rr_AverageAnnualReturnYear05 8.21%
10 Years rr_AverageAnnualReturnYear10 13.28%
Since Inception rr_AverageAnnualReturnSinceInception 7.13%
Inception Date rr_AverageAnnualReturnInceptionDate Apr. 01, 2002
SIIT LARGE CAP INDEX FUND | SIIT LARGE CAP INDEX FUND - CLASS A  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.05%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.08%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.13%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 13
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 42
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 73
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 166
Annual Return 2009 rr_AnnualReturn2009 28.60%
Annual Return 2010 rr_AnnualReturn2010 16.07%
Annual Return 2011 rr_AnnualReturn2011 1.47%
Annual Return 2012 rr_AnnualReturn2012 16.32%
Annual Return 2013 rr_AnnualReturn2013 33.12%
Annual Return 2014 rr_AnnualReturn2014 13.21%
Annual Return 2015 rr_AnnualReturn2015 0.90%
Annual Return 2016 rr_AnnualReturn2016 12.05%
Annual Return 2017 rr_AnnualReturn2017 21.59%
Annual Return 2018 rr_AnnualReturn2018 (4.80%)
Label rr_AverageAnnualReturnLabel Return Before Taxes
1 Year rr_AverageAnnualReturnYear01 (4.80%)
5 Years rr_AverageAnnualReturnYear05 8.18%
10 Years rr_AverageAnnualReturnYear10 13.27%
Since Inception rr_AverageAnnualReturnSinceInception 7.08%
Inception Date rr_AverageAnnualReturnInceptionDate Apr. 01, 2002
SIIT LARGE CAP INDEX FUND | SIIT LARGE CAP INDEX FUND - CLASS A | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 (8.62%)
5 Years rr_AverageAnnualReturnYear05 6.12%
10 Years rr_AverageAnnualReturnYear10 11.87%
Since Inception rr_AverageAnnualReturnSinceInception 6.09%
SIIT LARGE CAP INDEX FUND | SIIT LARGE CAP INDEX FUND - CLASS A | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and Sale of Fund Shares
1 Year rr_AverageAnnualReturnYear01 (0.14%)
5 Years rr_AverageAnnualReturnYear05 6.23%
10 Years rr_AverageAnnualReturnYear10 10.91%
Since Inception rr_AverageAnnualReturnSinceInception 5.36%